The invention relates of generally to the field of navigational systems, and is directed particularly to a new and improved TACAN/DME data link system which incorporates full navigation and communication functions on existing TACAN/DME equipment.
An airborne vehicle must frequently communicate with a ground station for purposes of relaying messages and obtaining data. Such communication can be performed on a separate radio frequency transmitter and receiver used solely for that purpose. This requires, however, the installation of additional airborne and ground equipment which represents an undesirable factor in terms of cost and weight. This is especially the case in certain modern airborne vehicles, such as unmanned drones, where compactness is desired.
One suggestion is that such communication could be performed by utilizing the transmitter and receiver carried aboard the airborne vehicle, such as, for example, the navigational equipment, for other purposes. One such navigational system that could be employed is the airborne TACAN/DME system found in wide airborne use, both civil and military. This system employs a pulse code format and has been assigned 252 crystal controlled channels centered around 1,000 MHz.
The possibility of communicating over a TACAN/DME system, based on the pulse nature of the system, has been investigated by several groups. For example, ITT has built a TACAN/DME data link system that permitted communication with multiple aircraft (see Electrical Communication, Vol. 34, Sept. 1957). The ITT system operated on a single frequency channel such that the aircraft would communicate only with the ground station from which it received its navigational data. The communication data was interleaved with the navigational data on the assigned and fixed frequency of the navigational ground station. A system based on this concept was built and flight tested but was never implemented for operational use.
A basic disadvantage of the ITT TACAN/DME system was that it required interleaving the navigation and communication functions on the same frequency. This required a special signal format to prevent interference between the communication and navigational function. This modified signal format, in turn, required extensive, complicated and costly modifications of the communication equipment. In fact, the airborne or ground based TACAN/DME equipment used to perform the navigation function required equipment modifications to restrict its random operating characteristic. The equipment modification was necessary for all units within the system, even those that did not supply or receive communication data to ensure compatability with the new signal format. The use of a special signal format to permit sharing of the same frequency channel also precluded a fully random operation of the navigational system. As a result, the number of aircrafts that could operate within the system was severely limited.
Recently, proposals have been advanced to add a more limited communication function to the TACAN/DME system. One such system is termed DDBS (i.e., Digital Data Broadcast System). Ground station transmissions in this system are coded to radiate a limited amount of data, such as station identity and station location. No further air-to-ground transmissions are provided. In fact, the data link is quite limited and, in many respects, is merely an extension of the limited data transmission capability that occurs with existing TACAN/DME ground stations, such as providing a station identifying code.
Another system for a limited air-to-ground TACAN/DME data link system has also been proposed (see IEEE 1980 PLANS Record, VDME-Based Azimuth System, p. 216-Horst Vogel et al). This system is also described in U.S. Pat. No. 3,680,117. In response to an interrogation pulse, the system's ground station irradiates a series of communications pulses in addition to the normal ranging pulses. These communications pulses immediately follow the ranging pulses and contain communication information. The communication pulses are identified by the airborne unit which sent the interrogation pulses since they are in a timed relationship with the airborne units. In general, these communication pulses convey information regarding the arrival angle of the radiated signal of the airborne interrogator and, as such, provide a form of navigational data. This system, however, doesn't have the capability of permitting the ground station to identify the airborne interrogator. Moreover, it merely provides uplink information; downlink communication is not offered. This system, like other prior art systems, provides a very limited data link capability.
In summary, with the prior art systems it was not possible for existing and unmodified TACAN/DME equipment to receive navigational data on a frequency channel used for both navigation and communication without interferring with the navigation function. Adding of the communication function, not only required extensive and complicated modifications in the communication equipment to comply with a new signal format, but also affected the operational characteristics of the existing equipment. The added communication function generated a substantial decrease in the navigational capability of the system, particularly in the number of aircraft that could obtain TACAN/DME range information. These systems did not solve the practical problems of fully incorporating a dual navigation and communication capability without adversely affected the existing navigational functions. By utilizing some of the TACAN/DME navigational pulses for communication, the prior art systems affected a decrease in the number of pulses available for navigation and, therefore, created a substantial decrease in the navigational capacity of the system.